The present invention relates to a biaxially oriented polyester film, more particularly to a laminated biaxially oriented polyester film which has easy tearability in at least one direction, excels in antistatic properties, and is useful as a packaging material for foods such as confectionary, pickles, retort pouch foods, etc., pharmaceuticals and such.
Since biaxially oriented polyester films represented by polyethylene terephthalate film are excellent in mechanical properties, electrical properties, chemical resistance, dimensional stability, etc., they are used as base for a variety of commercial products in many fields of industry such as information recording materials, capacitors, packaging materials, printing plates, insulators, photographic films, etc. In the recent years, polyester films have been popularly used as various types of packaging film, and amid the growing recognition of importance of environmental problems, it is expected that the scope of their use will be further expanded in view of their capability to prevent environmental pollution at the time of waste disposal.
The packaging polyester films find their way into many uses, and they are needed in various fields of industry such as foods, electric and electronic parts, machinery, equipment, building materials, chemicals, etc.
Various types of packing or wrapping bags utilizing plastic films, particularly those made by laminating a biaxially oriented plastic film and a sealant film, are popularly used for packaging foods, medicines and sundry goods, but these packages have the problem that when it is tried to rip them open, they may not be ripped up smoothly and linearly, causing a possibility of scattering the contents and consequent soiling of the user""s clothes when the packages are opened.
The packages using a plastic film having antistatic properties are also commonly used for packaging or wrapping powders, but these packages also involve the problems; some of them are bad in tearability by hand while others might be torn up in the wrong direction to cause scattering of the contents.
As a result of the present inventors"" earnest study to solve the above problems, it has been found that a film having a layer with a specific sea island structure and also possessing a specific surface resistance can solve the above problems.
The present invention has been attained on the basis of the above finding.
An object of the present invention is to provide a laminated polyester packaging film which can be ripped open with ease and linearly with no fear of causing scattering of the contents and also capable of preventing adhesion of powders.
To attain the above aim, in the first aspect of the present invention, there is provided a biaxially oriented polyester film having at least one layer of a sea island structure which satisfies the following formulae (1) and (2) at the same time, said film having a specific surface resistance of not more than 1xc3x971015 xcexa9:
50xe2x89xa6X/Yxe2x80x83xe2x80x83(1) 
5xe2x89xa6Nxe2x89xa6200xe2x80x83xe2x80x83(2) 
wherein X is the mean value of length (xcexcm) of the islands in the longitudinal direction of the film; Y is the mean value of length (xcexcm) of the islands in the width direction of the film; and N is the number (per xcexcm2) of the islands existing in the section of the film in its width direction.
The invention will be described in detail below.
The xe2x80x9cpolyestersxe2x80x9d referred in the present invention are the polymers having ester groups obtained from polycondensation of dicarboxylic acids and diols or hydroxycarboxylic acids. Examples of the dicarboxylic acids usable for the said polycondensation include terephthalic acid, succinic acid, isophthalic acid, adipic acid, azelaic acid, sebacic acid, dodecanoic diacid, 2,6-naphthalenedicarboxylic acid, and 1,4-cyclohexanedicarboxylic acid. Examples of the diols include ethylene glycol, 1,3-propanediol, 1,6-hexanediol, 1,4-butanediol, diethylene glycol, triethylene glycol, neopentyl glycol, 1,4-cyclohexanedimethanol and polyethylene glycol. Examples of the hydroxycarboxylic acids include p-hydroxybenzoic acid and 6-hydroxy-2-naphthoic acid.
Typical examples of such polymers are polyethylene terephthalate and polybutylene terephthalate. These polymers may be homopolymers or ones obtained by copolymerizing a third component.
Polyester D used in the present invention is a polyester obtained from a polycondensation reaction of a polyester B comprising a dicarboxylic acid and a glycol and a dehydrated condensate C mainly comprising a glycol. It is preferably one obtained from a polycondensation reaction of a polyester B comprising a dicarboxylic acid containing not less than 50 mol % of an aromatic dicarboxylic acid and a glycol containing not less than 50 mol % of a C3-C10 glycol and a dehydrated condensate C mainly comprising a C3-C10 glycol. Its preparation method, however, is not specified. Examples of the polyester B include polyethylene terephthalate, polypropylene terephthalate and polybutylene terephthalate. Examples of the dehydrated condensate C include polyethylene glycol, polytrimethylene glycol and polytetramethylene glycol.
The film of the present invention is a biaxially stretched film in view of high strength and excellent dimensional stability.
In order to prevent adhesion of the contents to the package, the film of the present invention needs to have antistatic properties. For this purpose, the polyester film of the present invention has a specific surface resistance of not more than 1xc3x971015 xcexa9, preferably not more than 5xc3x971012 xcexa9, more preferably not more than 5xc3x971011 xcexa9. When the specific surface resistance of the film is more than 1xc3x971015 xcexa9, the material contained in the package, especially powder, tends to adhere to the package.
As methods for affording antistatic properties to the film, there is exemplified a method in which an antistatic agent is incorporated in the film or the film is coated with an antistatic agent.
It is possible to use any type of antistatic agent as far as it is capable of reducing specific surface resistance. Examples of such antistatic agent include glycerin fatty acid esters, polyoxyethylene alkyl ethers, polyoxyethylene alkylphenyl ethers, alkyl diethanolamines, hydroxyalkyl monoethanolamines, polyoxyethylene alkylamines, alkyl diethanolamides, alkyl sulfonates, alkylbenzenesulfonates, alkyl phosphates, tetraalkylammonium salts, alkylbetains, alkylimidazoliumbetains and the like. Of these antistatic agents, alkyl sulfonates are preferred.
In the film of the present invention, the mixing ratio (by weight) of polyester D/polyester A falls within the range of 30/70 to 5/95, preferably 25/75 to 10/90, more preferably 20/80 to 10/90. When the ratio of polyester D exceeds 30% by weight, there is a tendency for the film to lower in mechanical strength and/or to increase in thickness unevenness, which may make the film impracticable.
Polyester D used in the present invention is preferably a melt mixed product of a polyester B and a dehydrated condensate C with its repeating units comprising mainly a C3-C10 glycol, in which the polyester B/dehydrated condensate C mixing ratio preferably falls within the range of 55/45 to 98/2, more preferably 5/95 to 30/70, especially 10/90 to 25/75. When the ratio of dehydrated condensate C is less than 2% by weight, insular dispersion of polyester D in polyester A matrix may not take place to badly deteriorate linear tearability of the film. On the other hand, when the ratio of the said condensate exceeds 45% by weight, the obtained film may be reduced in mechanical strength.
For affording easy and linear tearability to the film, it is necessary to effect insular dispersion of polyester D in at least one layer of the film, and the state of such insular dispersion of polyester D needs to satisfy the following formulae (1) and (2) at the same time:
50xe2x89xa6X/Yxe2x80x83xe2x80x83(1) 
5xe2x89xa6Nxe2x89xa6200xe2x80x83xe2x80x83(2) 
wherein X is the mean value of length (1 xcexcm) of the islands in the longitudinal direction of the film; Y is the mean value of length (xcexcm) of the islands in the width direction of the film; and N is the number (per xcexcm2) of the islands existing in the section of the film in its width direction.
Thus, in case where the mean values of length in the longitudinal and width directions of the islands observed on the section in the longitudinal direction and the section in the width direction of the film are supposed to be X (xcexcm) and Y (um), respectively, if X/Y is less than 50, linear tearability of the film may be bad. Also, when N is less than 5 (per xcexcm2), linear tearability may be badly deteriorated, and when N is greater than 200 (per xcexcm2), the dispersed polyester D is in excess, making it difficult to rip the film linearly.
A process for producing the film of the present invention is described concretely below, but the following instance is not limitative to the present invention and it is possible to use other methods as far as they are capable of producing the described effects of the present invention.
The respective polyester chips dried by a known technique are supplied to a melt extruder where the chips are heated to a temperature above the melting points of the respective polymers and thereby melted. Then the molten polymers are coextruded from a die onto a rotary cooling drum whereby the molten polymers are rapidly cooled to a temperature below the glass transition temperature and solidified to obtain a non-oriented sheet of a substantially amorphous state. In the above operation, in order to improve flatness of the sheet, it is preferable to enhance adhesion between the sheet and the rotary cooling drum. In the present invention, the electrostatic pinning method and/or the liquid coating method are preferably used for this purpose. In case of using an antistatic agent, it may be previously blended in the polyester chips.
In the present invention, the thus obtained sheet is stretched in the biaxial directions to make a film. Stretching is carried out under the following conditions. First, the said non-stretched sheet is stretched preferably 2 to 6 times in the machine direction at 70 to 145xc2x0 C. to make a monoaxially (longitudinally) stretched film, then further stretched 2 to 6 times in the transverse direction at 90 to 160, and then subjected to a heat treatment at 150 to 250% for 1 to 600 seconds. In this operation, the film is preferably relaxed 0.1 to 20% in the machine direction and/or the transverse direction at the highest temperature zone of the heat treatment and/or the cooling zone at the terminal of the heat treatment. Also, if necessary, the produced film may be subjected to re-stretching in the machine direction and/or the transverse direction.
It is possible to obtain a sea island structure satisfying the formulae (1) and (2) by properly adjusting the stretch ratio. For instance, in case where the value of X/Y of the formula (1) is not more than 50 at the stage of the sheet, the stretch ratio in the longitudinal direction is enlarged while that in the transverse direction is lessened within the above-defined range. Also, in case where the number of the islands N of the formula (2) is too large at the stage of the sheet, the stretch ratio in the longitudinal direction may be raised to enlarge X while reducing the number of the islands per unit area to satisfy the formula (2). It is also possible to adjust the number of the islands of the layer having the sea island structure by controlling the extruding temperature at the time of coextrusion or the degree of kneading. According to circumstances, when the extruding temperature is raised and kneading is conducted sufficiently, polyester D is uniformly dispersed in polyester A, which leads to a reduction of the size of the islands, resulting in an increase of the number of the islands. It is further possible to adjust the number of the islands at the sheet stage by controlling the speed of the take-up rolls and the die slit interval. Thus, by properly selecting the coextruding and/or stretching conditions, it is possible to make a laminated film having a layer of a sea island structure satisfying the formulae (1) and (2).
In the process of producing the polyester film according to the present invention, it is possible to incorporate a so-called in-line coating step in which the sheet just after stretched in the machine direction is coated before entering a tenter for transverse stretching and dried in the tenter for improving the specific properties such as adhesion, antistatic properties, weather resistance or surface hardness, provided that such an operation won""t be prejudicial to the purport of the present invention. Also, after production of the laminated film, it may be subjected to various types of off-line coating. It is possible to provide antistatic properties and adjust specific surface resistance of the film by such coating. Coating may be applied on one side or both sides of the film. Both water-based and solvent-based coating materials can be used for off-line coating, but a water-based or water-dispersed type coating material is preferably used for in-line coating.
In the polyester film of the present invention, it is possible to mix other types of thermoplastic resin such as polyethylene naphthalate and polytrimethylene naphthalate, and additives such as ultraviolet absorber, antioxidant, surfactant, pigment, fluorescent brightener, etc., within limits not affecting the effect of the present invention.
As described above, the present invention provides a laminated polyester film for packaging which allows easy and linear rip-opening of the package to eliminate the fear of causing scattering of the contents on opening of the package and is also proof against adhesion of powders.